1. Field of the Invention
The present invention relates to a propeller system for installation in aircrafts, and more particularly to an electric pitch control apparatus for a variable-pitch propeller.
2. Description of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 60-76499 is a pitch control apparatus for a variable-pitch propeller which is designed to control a pitch angle of the propeller blade and a rotational number of the propeller in accordance with a mach number, an altitude, an atmospheric temperature and an output power of the prime engine in flight of the aircraft to thereby maximize the operation efficiency of the propeller. Practically, the pitch angle of the propeller blade is controlled by an electro-hydraulic control device which is arranged to control a supply amount of operation fluid in accordance with a difference between a target rotational number and an actual rotation number of the prime engine.
In such a conventional pitch control apparatus, a control value of the propeller pitch is properly determined such that the actual rotational number is adjusted to coincide with the target rotational number. In the case that the electro-hydraulic control device was used for a long period of time, however, there will occur a difference between the target pitch control amount and the actual pitch control amount due to a secular change of the control device. Even if the electro-hydraulic control device was operated without any secular change, the difference between the target and actual pitch control amounts will occur if the flow amount of operation fluid changes due to fluctuation of the fluid temperature. For these reasons, an accurate control of the propeller pitch may not be effected.